This invention relates to an image pick up apparatus, and more particularly is suitable for application, for example, in a television (TV) camera system.
Some of conventional TV camera systems involve the use of a CCD (charge coupled device) imaging element.
To be specific, as illustrated in FIG. 1 , video information L0 condensed through a lens 2 at first undergoes an irradiation of a CCD imaging element 3 in such a CCD camera system 1. A video detection signal S0 obtained as a result of this is inputted to a CDS (correlated double sampling) circuit 4.
The CDS circuit 4 sample-holds the video detection signal S0 at a predetermined timing and generates a video signal S1. This video signal S1 is transmitted to a signal processing circuit of the subsequent stage.
Inputted via a driver 6 to the CCD imaging element 3 are driving pulses consisting of first, second and last horizontal transfer pulses H1, H2, LH and reset pulses RP which are transmitted from a timing generation circuit 5 as disclosed in Japanese Patent Laid-Open Publication No. 61-273079. The video information L0 is read at timings corresponding to a television system, thereby generating the video detection signals S0.
Inputted also to the CDS circuit 4 are first and second sampling pulses SP1, SP2 at timings corresponding to the horizontal transfer pulses H1, H2, LH and the reset pulses RP.
The CDS circuit 4 thereby sampling-holds the respective video detection signals S0 at timings of a signal level and a reference level. Subtraction thereof is effected, whereby the video signals S1 composed of signal level components of the video detection signals S0 can be generated.
If the thus constructed CCD camera system 1 is employed as a TV camera based on, e.g., a high-definition television system, for instance, shading or the like takes place because of the noise components due to entrances of the driving pulses H1, H2, LH, RP transmitted from the timing generation circuit 5. This results in a deterioration of an S/N ratio. The video signal S1 declines in terms of image quality.
The shading noise due to the horizontal transfer pulses H1 and H2 is generated as shown in FIGS. 2(A) to 2(C).
There may be no trouble in the case where the horizontal transfer pulses H1 and H2 have the identical level and the exactly opposite phase each other, because the horizontal transfer pulses entering into the CCD output signal cancel each other.
But actually, the phases of the horizontal transfer pulses H1 and H2 tend to have phase errors of rising edges on re-generating after the horizontal blanking period T.sub.HBLK as shown in FIGS. 2(A) and 2(B). AS the result the shading noise S.sub.SHD is generated as shown in FIG. 2(C).
This problem arises even in the standard television system of NTSC, etc. As compared with the high-definition television system, however, frequencies of the driving pulses H1, H2, LH, RP are low enough so that real time correction is unnecessary. This can be sufficiently corrected even in the signal processing circuit of the subsequent stage.
In the case of the TV camera of the high-definition television system, however, the frequencies of the driving pulses H1, H2, LH, RP increase corresponding to the horizontal frequencies. There exists such a problem that no correction can be made by the signal processing circuit of the subsequent stage unless the correction is effected in real time.